The present invention is related generally to electronics semi-conductor processing. More specifically, the present invention is related to controllable oxidization of the side edges of die at the wafer level, and in particular the passivation of the side edges of aluminum bearing Group III-V semiconductor layers in electronic or electro-optical devices, such as VCSEL chips.
Semiconductor lasers are commonly used in modern technology as a light source in various devices, including communication devices, such as fiber optic transmitters, and compact disc players. A typical semiconductor laser is a double heterostructure with a narrow bandgap, high refractive index layer surrounded on opposed major surfaces by wide bandgap low refractive index layers. The low bandgap layer is termed the xe2x80x9cactive layerxe2x80x9d and the bandgap and refractive index differences serve to confine both charge carriers and optical energy to the active layer or region. Opposite ends of the active layer have mirror facets which form the laser cavity. The cladding layers have opposite conductivity types, and when current is passed through the structure, electrons and holes recombine in the active layer to generate light.
Surface-emitting, rather than edge-emitting lasers have been developed. One surface-emitting laser is a xe2x80x9cvertical cavity surface emitting laserxe2x80x9d (VCSEL). Vertical Cavity Surface Emitting Lasers offer numerous performance and potential producibility advantages over conventional edge emitting lasers. These include many benefits associated with their geometry.
Surface emitting devices can be fabricated in arrays with relative ease while edge emitting devices cannot be as easily fabricated. An array of lasers can be fabricated by growing the desired layers on a substrate and then patterning the layers to form the array. Individual lasers may be separately connected with appropriate contacts. Such arrays are potentially useful in such diverse applications as, for example, image processing, inter-chip communications (i.e., optical interconnects), and so forth. Typical edge-emitter lasers are turned on and off by varying the current flow through the device. This often requires a relatively large change in the current through the device which is undesirable. In comparison, surface-emitting lasers often require lower drive current, and thus the change of current to switch the VCSEL need not be as large.
High-yield, high performance VCSELs have been demonstrated, and exploited in commercialization. Surface-emitting AlGaAs-based VCSELs are producible in a manner similar to semiconductor integrated circuits, and are amenable to low-cost high-volume manufacture and integration with existing electronics technology platforms. Moreover, VCSEL uniformity and reproducibility have been demonstrated using a standard, unmodified commercially available metal organic vapor phase epitaxy (MOVPE) chamber and molecular beam epitaxy (MBE) chamber giving very high device yields.
VCSELs typically have an active region with bulk or one or more quantum well layers. On opposite sides of the active region are mirror stacks which are formed by interleaved semiconductor layers having properties, such that each layer is typically a quarter wavelength thick at the wavelength (in the medium) of interest thereby forming the high-reflectance mirrors for the laser cavity. There are opposite conductivity type regions on opposite sides of the active region, and the laser is typically turned on and off by varying the current through the active region.
VCSELs may have multiple aluminum bearing Group III-V layers. In particular, the VCSEL may have AlAs layers and AlGaAs layers. The aluminum bearing layers are protected from the environment in a vertical direction by the top surface, which can include one or more surface passivation layers. The aluminum bearing layers typically are exposed to the environment at the edges or side face surfaces, particularly after the wafer has been cut into individual die. The aluminum bearing edges can oxidize when the chip is placed into service in an oxidizing environment. The environmentally induced oxidation can cause unreliable oxidation, from the edge inward, of the aluminum bearing layers. If left unchecked, this lateral oxidization can sometimes reach the VCSEL device itself, thereby reducing performance or even preventing operation altogether. To prevent or inhibit such lateral oxidation of the aluminum bearing layers, the chip is commonly mounted in a hermetically sealed package. Hermetically sealed packages have a number of limitations. First, hermetically sealed packages can be relatively expensive, which increases the cost of the device. Second, hermetically sealed packages can be relatively bulky, which increases the space needed to mount the device on a circuit board or multi-chip package, both of which are undesirable.
What would be desirable is a method for sealing the edges of chips, particularly VCSEL chips, prior to cutting the chips from the wafer. VCSEL chips having sealed edges may not require a hermetically sealed package.
The present invention provides methods for passivating the edges or side faces of aluminum bearing Group III-V layers in die such as VCSEL die. In one illustrative method, a semiconductor wafer having multiple VCSEL die separated by streets is provided. The VCSEL die preferably have aluminum bearing layers disposed a depth under the laterally disposed top wafer surface. Channels are cut into the streets to a depth sufficient to expose the side edges of the deepest aluminum bearing lateral layer. The channels are then exposed to an oxidizing environment such as steam, oxygen, or any other suitable oxidizing environment.
The oxidizing atmosphere oxidizes the aluminum bearing layers into aluminum oxide layers, forming a stable native oxide passivating layer which is resistant to further oxidation by the intended working atmosphere. Some of the non-aluminum bearing layers of a VCSEL, including layers formed from GaAs, InGaAs, InGaAsN, GaAsN, GaAsP, InGaAsP, etc., may also oxidize, but at a reduced rate. In one embodiment, the aluminum bearing layers are oxidized about 10 to 15 microns into the channel wall. After the aluminum bearing layers are sufficiently oxidized, the wafer is removed from the oxidizing atmosphere. The heat (xcx9c440xc2x0 C.) results in almost immediate drying.
The wafer can then be cut into discrete die with a blade using methods well known to those skilled in the art. The wafer cuts are preferably made along the channels without destroying the intentionally oxidized layers. Preferably, the channel walls are oxidized prior to cutting the wafer into individual die. Alternatively, however, it is contemplated that the wafer may be cut into individual die before the side walls are oxidized. In this latter embodiment, no channels need be formed.
The present invention also includes discrete VCSEL chip having passivated aluminum bearing layers around the chip periphery. In particular, the present invention includes VCSEL chips having AlAs and AlGaAs layers where the layer regions near the chips side edges or side faces are oxidized to form a stable native oxide. The native oxide layers are believed to include anhydrous forms of aluminum hydroxides and aluminum oxide hydroxides such as alpha-Al2O3, gamma-Al2O3, diaspore, and boehmite.
Chips made according to the present invention have side regions that are resistant to further oxidation by the environment. In particular, VCSEL chips made according to the present invention are believed suitable for direct mounting to boards without hermetically sealed packaging.